This invention relates to liquid crystal display cells and in particular to the provision of molecular alignment layers on the interior major surfaces of the cell walls for promoting parallel homogeneous alignment of the liquid crystal molecules in contact with such layers.
For some years it has been known that parallel homogeneous molecular alignment can be achieved by rubbing a glass sheet with a suitable tissue. It was found that this rubbing left some residue on the glass, probably a grease, and that it was this rubbed residue which provided the alignment. Since then it has been common practice to apply a specific material such as polyvinyl alcohol to the glass, and to rub this to provide the required alignment. This works satisfactorily for cells which are sealed using standard plastic sealing technology, but neither the grease nor materials such as polyvinyl alcohol are able to withstand the sort of temperature required to form the glass frit seals that are required for high reliability devices. Glass frit sealing removes the alignment. One approach to this problem has been to replace the polyvinyl alcohol with a plastic material that can withstand higher temperatures such as polyimide which in suitable circumstances can withstand brief heating to a temperature of around 430.degree. C. This is at the borderline of the temperature range required for glass frit sealing. A typical glass frit seal is fired at around 475.degree. C.
Parallel homogeneous alignment that is fully compatible with glass frit sealing temperatures has previously been achieved by oblique evaporation of a suitable material such as silicon monoxide. The principal drawback of this approach is cost. The process requires quite a good quality vacuum, and requires a relatively large vacuum chamber in order to give an adequate spacing between target and source to minimize the variation in angle of deposition over the surface of the target.